Post partum hemorrhage, a major cause of maternal morbidity and mortality in childbirth has not been quantitatively assessable due to the lack of a noninvasive instrument for the measurement of maternal blood loss. A noninvasive, convenient and inexpensive method is needed to detect and assess the degree of post partum blood loss so that appropriate interventions can be applied. Investigations that will lead to the development a measurement system that will meet this need is proposed herein. The proposed method uses conventional indicator dilution theory, substituting the 18O2 stable oxygen isotope with a combination of inert gases for the more commonly used dyes and radioactive tracers. Using these tracers, total blood volume, Vb, hemoglobin, Hb, and blood loss, BL, will be measured. Preliminary experiments in the proposing laboratory have demonstrated the feasibility of the proposed method and have brought about the acquisition of most of the special purpose instrumentation that will be needed. In this study all the proposed measurement techniques will be conclusively validated and the methods will be refined to meet the accuracy criteria necessary for clinical utility in obstetrics practice as well as research. Experiments in dogs will be conducted in which degrees of blood loss will be imposed by repeated exsanguination and retransfusion while measurements of Vb, Hb, and BL are made using the tracer methods. Subsequently human volunteers will be used to validate these measures within their expected physiologic ranges. BL measurement sensitivity will be observed in human blood donors. Also these donors will be evaluated for hemoglobin changes after donation. Finally demonstration will be made of the ability to apply the method to women during labor. Hardware and software engineering for the design of a device will be relegated to Phase II. PROPOSED COMMERCIAL APPLICATIONS: Since a non-invasive blood volume monitoring system does not presently exist, a commercial device incorporating the methods developed in this program will fill an immediate need in the clinical practice of obstetrics.. Essentially the same device will fill an important role in other blood loss monitoring situations including surgery and trauma.